Porroprism

A porroprism is a type of optical prism that deviates a beam of light by 90 degrees while reinverting the image both vertically and horizontally. It is composed of a rectangular prism with specific angles cut into its surfaces. Light enters one face, reflects off an internal surface, and exits through another face. The precise geometry of the porroprism ensures that the light undergoes two internal reflections. The first reflection flips the image vertically, and the second reflection flips it horizontally. This double inversion results in an erect, non-reversed image as seen by an observer.

Porroprisms are commonly found in binoculars, where they are used to produce an upright image without requiring long objective lenses. The characteristic shape of binoculars with porroprisms is often described as having a "dog-leg" or angled appearance due to the offset of the objective lenses and eyepieces, which is necessary to accommodate the prism's internal path. While effective, porroprisms can be bulkier than other prism designs like roof prisms, which achieve a similar image inversion in a more compact form. Despite their size, porroprisms are valued for their high light transmission and the quality of the erect image they produce.